(1) A conventional apparatus which measures transfer functions of distributed constant networks without having inner signal sources will be described in the following:
A prior art transfer function measurement apparatus is shown in FIG. 2. A distributed constant network 111 under observation is composed of a passive element 113 and an active element 114 connected on a substrate 112 with a distributed constant line 115 like a microstrip line. An input terminal 116 and an output terminal 117 which input and output signals are installed in the network 111. To measure the transfer function between the input terminal 116 and the output terminal 117, a test signal is input from a network analyzer 118 to the input terminal 116. The output signal from the output terminal 117 is input to the network analyzer whereby the transfer function is measured. For measuring a transfer function of a part of the circuit of the network 111, a test signal is input from the network analyzer 118 to the input terminal 116. A contact type probe 119 contacts the part of the network 111 to pick up a signal. The signal picked by the probe 119 is input to the network analyzer 118 and the transfer function is measured. When the location in the network 111 to measure a transfer function is known in advance, a power splitter 121 is connected at the location. The network 111 is so composed beforehand that a test terminal 122 is connected with the power splitter 121 and the signal is input and output through the test terminal 122. The test signal is measured between the test terminal 122 and the input terminal 116, or between the output terminal 117 and the test terminal 122, and the transfer function is measured.
In this arrangement of the prior art, the probe 119 influences the network 111 and thus a correct measurement cannot be available because the probe 119 directly contacts the network 111 as mentioned above in the measurement of a part of the network 111, although a correct measurement is available for the measurement between the input terminal 116 and the output terminal 117. Moreover, when the test terminal 122 is used, a relatively large loss of signal power is caused because the power splitter 121 always exists in the network 111. Moreover, unwanted couplings between the power splitter 121 and other circuit parts may be caused and thus, effective production of the networks which perform a correct operation is difficult.
(2) In the following, a conventional signal observation apparatus is described, which measures, by a non-contact method, spectrums and waveforms of the signal in a distributed constant network having inner signal sources.
Prior art signal observation apparatus is shown in FIG. 5. A distributed constant network 211 to be tested is provided with passive elements 213-215, and oscillation circuits 216-217 on a substrate 212. These elements are connected by distributed constant lines 218. An input terminal 219 and an output terminal 220 are also provided. A contact type probe 221 contacts the location that is to be measured in the distributed constant network under observation 211, and a signal picked at the location is input to a measurement unit such as an oscilloscope 222 and the signal waveform of the observation point is displayed. Alternatively, the signal of the output terminal 220 is input to the measurement unit such as an oscilloscope 222 thereby a signal waveform is measured.
If appropriate, a power splitter 223 is connected with the point of the network 211 that is to be measured as shown in FIG. 5 and a test terminal 224 is also installed in advance in the network 211. A signal is input through the test terminal 224 to the measurement unit such as an oscilloscope 222.
When signals at the location other than the input or output terminals are observed, the contact type probe 221 affects the performance of the network. Thus, accurate measurement of the characteristics of the network 211 is not available in the above-mentioned contact method because the contact probe 221 directly contacts the network 211.
Further, in case where the test terminal 224 is fixed to the network 111, there is a disadvantage that a power splitter for the test terminal 224 causes a relatively large signal loss in the network 211. Furthermore, unwanted couplings in the signals will be caused in the power splitter 223, and thus, the network 211 may not operate as expected. Moreover, when locations in the network 211 other than the test terminals need to be measured, the contact type probe as noted above must be used.
(3) In the following, a conventional spatial electromagnetic wave analysis apparatus is described, by which information on a spatially polarized wave vector of an electromagnetic wave is obtained.
Measurement of unwanted electromagnetic waves radiated from electronic devices, or a measurement of electromagnetic wave propagation environment such as compound scattering by a structure, or electromagnetic wave propagation in an ununiform medium, etc., are made as follows.
Intensity of one polarization in a specific direction was examined by a unidirectional, single polarized antenna of narrow beam such as a Yagi antenna, a helical antenna, or a horn antenna, etc., by rotating the antenna in a direction of the beam radiation as an axis, turning an azimuth angle of the antenna or an elevation angle of the antenna.
In this conventional technology, only linearly polarized electromagnetic waves can be received in the past by the Yagi antennas. Only circularly polarized electromagnetic waves can be received by the helical antennas. Thus, polarization information observed is limited by the antenna used. In the actual observation space, electromagnetic waves of various polarization modes exist. To know this information, electromagnetic waves such as horizontally polarized waves, vertically polarized waves, right-handed polarized waves, and left-handed polarized waves must be measured and the results must be analyzed totally. Moreover, a spatial distribution image is not easily obtainable in the conventional apparatus.
(4) A conventional holographic radar is described in the following, which radiates electromagnetic waves to the space, receives reflected waves from the space at each observation point on a hologram observation plane, performs hologram reconstruction calculations, and then observes the reflecting bodies.
A prior art holographic radar is shown in FIG. 11. High frequency signals such as microwave or millimeter waves are supplied to an antenna 412 for electromagnetic-wave emission by a network analyzer 411. The high frequency electromagnetic waves are continuously radiated to an observation space. Interference waves in the reflected waves from two or more places are received by a receiving antenna 414 at each observation point (x, y) on a hologram observation plane 413 which is set up opposite to the observation space. The received signal is input to the network analyzer 411.
Transfer functions H(x, y, f) of the route of electromagnetic waves from antenna 412 for the electromagnetic-wave emission to the observation point (x, y) are obtained by the network analyzer 411. The receiving antenna 414 is moved to each observation point (x, y) on the observation plane 413 and the reflected waves are received at each observation point (x, y). Alternatively, the receiving antennas 414 can be arranged at each observation point (x, y) on the observation plane 413 like an array. These antennas are switched one by one on the observation plane 413, a signal of each observation point (x, y) is received, and is input to the network analyzer 411. That is, the reflected wave at each observation point (x, y) on the observation plane 413 is received by the scanning-antenna receiving means.
The frequency f of the radiated electromagnetic wave is changed step by step and a space transfer function H(x, y, f) of each frequency f is measured as mentioned above. For the transfer function H(x, y, f) thus obtained, hologram reconstruction calculation shown by the next equation is performed by a hologram reconstruction calculation unit 415. EQU I(U, v, r)=.intg..intg..intg.H(x, y, f) exp{-j2.pi.(ux.sub.-- vy-fr)}dxdydf(1)
where u is an azimuth angle and v is an elevation angle when the observation space is seen from the observation plane 413, and r is a distance from the observation plane 413.
The hologram reconstruction result I(u, v, r) is supplied to a three-dimensional display unit 416 and is displayed in three dimensions. For instance, I(u, v, r) is displayed in two dimensions of u and v plane, at first, when r is fixed. Next, I(u, v, r) is displayed while r is varied, and so forth.
In this conventional holographic radar, there is a problem that it is necessary to perform the hologram reconstruction calculation according to the above-mentioned equation and thus needs an extremely large amount of calculation because the equations (1) requires a triple integration.
(5) A conventional display apparatus used for displaying space propagation of the wave or a holographic radar which uses such waves is described in the following.
A three-dimensional real number space shape can be expressed by a two-dimensional computer graphic, for example, by a method in which a distance is expressed by limiting brightness, that is, by shadowing.
In the conventional technology, a three-dimensional space complex number shape like an electromagnetic wave which has not only intensity information but also phase information, polarization information, and axial ratio information, etc., cannot be expressed on a two-dimensional surface.